1. Field of the Invention
The present invention relates to a metal oxide insulating film containing fluorine and a metal, and a semiconductor device using this film.
2. Description of the Related Art
In order to assure an electric charge amount induced in a channel of an MISFET (a metal insulator semiconductor field effect transistor), a technique of reducing a film thickness of a gate insulating film to increase a capacitance has been conventionally adopted. As a result, a reduction in film thickness of a SiO2 film as a gate insulating film has been propelled, and a film thickness is currently about to reach a value that is far smaller than 1 nm. However, in the SiO2 film, a gate leakage current is increased, and power consumption cannot be suppressed because of dissipation of a standby energy For example, in a SiO2 film having a film thickness of 0.8 nm, a gate leakage current has reached 1 kA/cm2, thus the problem regarding power consumption is very serious.
Increasing a film thickness is effective for a reduction in power consumption. Therefore, there has been examined an insulating film that can assure an amount of electric charge by using a high-K dielectric material even though this film is thicker than a SiO2 film. As stable high-K dielectric materials, many metal oxides are known.
As insulating films having such characteristics, especially insulating films that are currently prospective, there are HfO2, ZrO2, silicate films using these materials (HfSiO, ZrSiO), LaAlO3 and others.
However, a high-K dielectric metal oxide is apt to contain an oxygen vacancy, and this oxygen vacancy freely moves. As a result, the following problems occur.
(1) The oxygen vacancy that moves freely serves as a trigger, precipitation of a crystal thereby readily occurs from an amorphous structure, and hence uniformity of insulating film characteristics cannot be guaranteed (a problem of phase separation and crystallization).
(2) A vacancy level due to the oxygen vacancy occurs in a band gap, and this becomes a source of a leakage current (a problem of leakage through the oxygen vacancy).
(3) A structural defect occurs with movement of the oxygen vacancy, and long-term reliability is deteriorated (a problem of reliability).
(4) When an electron is shifted from the oxygen vacancy, the energy is stabilized, and a large shift of a threshold voltage (Vth) occurs. A Fermi level of an electrode is fixed, Vth of a PMOS is increased by approximately 0.6 eV to be pinned (a Vth pinning problem in a PMOS).
These problems are problems relating to all insulating films, not just MISFETs. The problems related to the characteristics of an insulating film are of critical importance with regards to insulating films used in a metal-insulator-metal (MIM) capacitor sandwiched between metal electrodes, or a flash memory cell or a part of its laminated structure.
Meanwhile, a first gate insulating film, which is considered to be most important in the current situation, is a nitride (HfON, ZrON, HfSiON, or ZrSiON). However, introduction of nitrogen has the following many problems.
(1) Nitrogen is small in capability of pinning an oxygen vacancy, and a large amount of nitrogen must be introduced to suppress crystallization.
(2) A reduction in a band gap due to nitrogen is considerable. When a large amount of nitrogen is introduced, a band offset ΔEc with respect to a Si substrate on a conduction band side is reduced by nearly 0.5 to 1.0 eV. A band offset ΔEv with respect to a Si substrate of a valence band is also decreased by approximately 1.0 eV. A leakage current is considerably deteriorated as compared with an essential merit.
(3) A problem also occurs in long-term reliability due to nitrogen. With the introduction of nitrogen, the oxygen vacancy is also increased. However, as the oxygen vacancy cannot be sufficiently fixed, a structure is changed with movement of the oxygen vacancy over a long period of time. Further, when the structure changes, fixed charges or fixed polarization occurs, thus considerably deteriorating dielectric characteristics.
(4) A complex of nitrogen and an oxygen vacancy Vo (which will be referred to as “NVoN” hereinafter) goes readily to pieces. At this time, the fixed charges enter a dispersed state, and this becomes a factor of reduced mobility of a carrier traveling through a channel.
(5) When excessive introduction of nitrogen is present, negative charges are produced. That is because nitrogen excessively takes in electrons. Furthermore, the oxygen vacancy is apt to occur near nitrogen, depending on the process, and the oxygen vacancy Vo is charged to become positive. In this manner, electric charges caused due to introduction of nitrogen have a possibility of greatly fluctuating Vth. Moreover, since the fixed charges become scattered bodies, this provokes serious deterioration in mobility. When nitrogen is additionally introduced based on thermal diffusion after forming an HfSiON film, the oxygen vacancy Vo is produced. As a result, an insulating film having an increased internal positive electric charge amount is generated, thus deteriorating mobility. Although electric charges are compensated, the number of scattered bodies themselves is increased.
The second gate insulating film that appears promising is an aluminate thin film (e.g., HfAlO). Since Al has a very small ion radius, phase separation readily occurs. Since many oxygen vacancies are introduced when Al is adopted, it is known that phase separation is apt to occur through movable oxygen vacancies. In order to maintain an amorphous structure, a large amount of Al must be adopted. In this case, however, the lowest part of a conduction band is greatly affected, and a considerable reduction in ΔEc also occurs.
In regard to an improvement in reliability, a method of introducing F is proposed (see, e.g., JP-A 2002-299614). It is known that F terminates, e.g., a dangling bond of an interface of a substrate and an insulating film, and prevents reliability-deteriorating mechanisms, such as negative bias temperature instability of the interface (NBTI) or the like, from occurring, thereby improving reliability.
However, when fluorine is simply introduced into a film, fixed charges are produced, and a shift of Vth occurs, thereby making it difficult to control. This leads to a source of serious deterioration in mobility because of the fixed charges. Therefore, a method or a structure that increases an amount of fluorine near an interface required to improve reliability and prevents fluorine from being introduced into a film has been vigorously examined.
Further, when fluorine is present near an interface of a substrate and an insulating film, NBTI reliability is improved. Fluorine in the film does not have any effect on NBTI, and becomes a charge center (the fixed charge) as aforementioned. Therefore, fluorine is not introduced into the film. On the other hand, when nitrogen is present near an interface of an insulating film and an electrode film, it demonstrates an effect of preventing boron from coming through the interface. When nitrogen is present near an interface of a substrate and an insulating film, it provokes occurrence of fixed charges or distortion of an interface structure, and hence a structure in which nitrogen is apart from the interface of the substrate and the insulating film is good.
An oxide insulating film formed of, e.g., HfO2 involves a problem of precipitation of micro-crystals in a heating step in manufacture of a semiconductor device. Although introduction of nitrogen or Al is attempted to overcome this problem, the essential merit of this film is largely offset by the very high leakage current. It is therefore necessary to provide an insulating film that fixes or eliminates a movable oxygen vacancy, produces no micro-crystal precipitation and no reduction in a band, and avoids a band gap state.